The present invention relates to an aluminum alloy sheet for a lithographic printing plate. More particularly, the present invention relates to an aluminum alloy sheet for a lithographic printing plate that may be suitably surface-roughened by an electrochemical etching treatment and ensures a high productivity.
An aluminum alloy sheet is generally used as a support for a lithographic printing plate (including an offset printing plate). An aluminum alloy sheet used as such a support is surface-roughened in order to improve its adhesion to a photosensitive film and improve the water retention in a non-image area. In recent years, a method that roughens the surface of an aluminum alloy sheet used as a support by an electrochemical etching treatment has been increasingly developed due to excellent plate-making applicability (fitness), excellent printing performance and a continuous treatment capability using a coil material.
As an aluminum alloy sheet that can be relatively uniformly surface-roughened by electrolysis using an electrochemical etching treatment, an A1050 (aluminum purity: 99.5%) equivalent material or a material obtained by adding small amounts of alloy components to an A1050 equivalent material has been utilized. Several proposals have been made on such alloy components (see JP-A-2000-108534, for example).
In order to improve the plate wear of a printing plate, a printing plate using an aluminum alloy sheet as a support is subjected to exposure and development using a normal method, followed by a high-temperature heat treatment (burning treatment) to strengthen the image area. The burning treatment is generally performed at 200 to 290° C. for 3 to 9 minutes. An aluminum alloy sheet used as a support is required to exhibit heat resistance (burning resistance) which maintains the strength of the support during the burning treatment.
In recent years, the printing speed has increased along with the progress in printing technology so that stress applied to a printing plate mechanically secured on each side of a plate cylinder of a printer has increased. Therefore, a support having a high strength has been desired. If the strength of the support is insufficient, the secured portion of the support may be deformed or damaged, so that a printing variation or the like may occur. Accordingly, an increase in strength of the support is indispensable together with the burning resistance.
In order to satisfy the above requirements, attempts have been made to adjust the components added to an A1050 equivalent material (see JP-A-2005-15912). Attempts have also been made to adjust the components added to an A1050 equivalent material while adjusting the depth of oil pits in the sheet surface (see JP-A-2004-35936).
Such an aluminum alloy material for a lithographic printing plate has been produced by homogenizing an ingot, hot-rolling the homogenized product, cold-rolling the hot-rolled product while performing process annealing to form a recrystallized structure on the surface of the rolled sheet, and subjecting the cold-rolled product to secondary cold-rolling, thereby ensuring uniform pit formation during an electrochemical etching treatment and preventing streaks when forming a printing plate. However, since a decrease in productivity and an increase in production cost necessarily occur due to process annealing, an improved production method has been desired.
A method that obtains an aluminum alloy sheet for a lithographic printing plate by cold-rolling a hot-rolled product without performing process annealing has been proposed (see JP-A-11-335761). In this method, hot-rolling includes rough hot-rolling and finish hot-rolling. The rough hot-rolling start temperature is set at 450° C. or more. An aluminum alloy is subjected to rough hot-rolling at a rolling speed of 50 m/min or more, and a rolling reduction of 30 mm or more or a single-pass rolling reduction ratio of 30%. The rough hot-rolling finish temperature is set at 300 to 370° C. The finish hot-rolling finish temperature is set at 280° C. or more. The rolled product is then wound in the shape of a coil to control the recrystallization state of the surface of the sheet.
In order to omit process annealing, it is necessary that an aluminum alloy sheet has been recrystallized when wound in the shape of a coil after finish hot-rolling. In order to obtain uniform electrolytic surface-roughening characteristics, it is important that recrystallized grains are minute and uniform in the same manner as in a material subjected to process annealing, and the surface area of the sheet is uniformly recrystallized.